Radar level gauges (RLGs) are suitably used for making measurements of the level of products such as process fluids, granular compounds and other materials contained in a tank. An example of such a radar level gauge can include a transceiver for transmitting and receiving microwaves, a signal propagating device arranged to direct microwaves towards the surface and to return microwaves reflected by the surface to the transceiver, and processing circuitry adapted to determine the filling level based on a relation between microwaves transmitted and received by the transceiver.
The signal propagating device may be a directional antenna, adapted to emit free propagating electromagnetic waves into the tank and receive a reflection of those waves. Such an RLG is sometimes referred to as a non-contact RLG. The antenna may be adapted to a specific frequency band, and the currently most used frequency bands have center frequencies of around 6 GHz or 24 GHz.
In one conventional design the wave guide arrangement comprises a hollow wave guide which extends through the tank wall, which wave guide is filled with a dielectric filling member (plug) to prevent tank content from entering the wave guide. Sealing elements, such as o-rings or the like, are arranged around the plug to provide a process seal ensuring that the contents of the tank are not released into the outside environment. The process seal may be pressure tight.
The dielectric material in the filling member is selected to have suitable dielectric properties, and is also preferably hydrophobic, i.e. repelling to water. However, such materials, e.g. PTFE, are typically also relatively soft, and affected by elevated temperatures. Under conditions of varying temperatures, a soft dielectric filling member, such as a PTFE filling member, may therefore move in relation to the surrounding wave guide, which is typically made of steel. Such movement may cause the sealing provided by the sealing elements to be degraded, leading to imperfect tank sealing.
Further, during conditions of high temperature and high pressure (HTHP) in the tank, there is also a risk that a soft dielectric member, will be forced out of the wave guide, also through a relatively small opening (i.e. much smaller than the diameter of the dielectric member).